A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In order to be able to project ever smaller structures onto substrates, it has been proposed to use EUV radiation, which is electromagnetic radiation having a wavelength within the range of 10-20 nm, for example within the range of 13-14=nm. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm.
Radiation may be produced using plasma. The plasma may be created, for example, by directing a laser at a fuel, such as particles of a suitable material (e.g. tin), or a stream of a suitable gas or vapor, such as Xe gas or Li vapor. The resulting plasma emits radiation, e.g., EUV radiation, which is collected using a collector such as a mirrored collector, that receives the radiation and focuses the radiation into a beam. The radiation emitting plasma and the collector may together be considered to comprise a radiation source. A radiation source which generates plasma by directing a laser onto fuel particles, as described above, is often referred to as a laser produced plasma (LPP) source.
In addition to radiation, the generation of plasma by a plasma radiation source produces contamination in the form of particles, such as thermalized atoms, ions, nanoclusters, and/or microparticles. The contamination is output, together with the desired radiation, from the plasma towards the collector and may cause damage to the collector and/or other parts of the radiation source, and/or other parts of the lithographic apparatus.
It is desirable to reduce the incidence of contamination on surfaces of a radiation source and/or other parts of a lithographic apparatus.